The evolution and biomechanics of dinosaurian tails

• Main Author: Pittman, M. D.
• Published: University College London (University of London) 2012
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.587636

The tail serves a range of functions including balance. Intervertebral joint stiffness, a measure of spinal rigidity, is useful for understanding its capability to perform basic functions, such as support against gravity. Dolphin joints of low and high stiffness are correlated with distinct vertebral morphologies. These principles apply for qualitatively assessing dinosaurian joint stiffness, because they relate Newtonian mechanics to morphology but this was not validated in the living relative, Crocodylus. Morphometric and qualitative phylogenetic data were gathered from theropod, sauropodomorph and thyreophoran dinosaur tail specimens, and then mapped on composite phylogenetic trees to reconstruct nodal joint stiffnesses. Initially, the tails of non-avian theropods were supported against gravity via static tail joint stabilisation. However, these tails became more dominantly stabilized by dynamic properties, as dorsoventral and lateral joint stiffness decreased towards the paravian node. In birds, the tail joints became stiffer dorsoventrally and laterally between Avialae and the crown group, enabling larger muscular forces to be produced. This change potentially allowed birds to utilise larger lift forces. The evolution of quadrupedality in sauropodomorphs and thyreophorans reduced the tail’s importance for balance and manoeuvrability, allowing alternative functions to evolve. The sauropods Shunosaurus and Mamenchisaurus appear to have had unique clubbing behaviours because of differences in joint stiffness, as well as tail club and proximal caudal articular surface morphologies. Along the stegosaurian and ankylosaurine lineages, the tail joints stiffened dorsoventrally and laterally, which improved tail support against gravity, and required larger muscular forces for weapon wielding. Therefore, not all dinosaurian tail weapons evolved in the same way. This thesis makes an important step in the understanding of dinosaurian tail function. However, future validation experiments would help to improve the accuracy of joint stiffness reconstructions in fossil taxa. Greater taxon sampling would also help to test and broaden the insights of this thesis.